Polylactic acid or poly lactide (PLA) is a kind of eco-friendly and biodegradable material, which can be used for preparing various kinds of medical materials, such as implantable hard tissue repair material, surgical suture, targeted and controlled release drug carrier, etc. In addition, polylactic acid can also be applied to the preparation of various biodegradable plastic products, such as films, fibers, packaging materials, etc. The production of commercially available polylactic acid is performed via catalyzed ring-opening polymerization using lactide as the monomer. Thus, the synthesis of lactide has an important significance for the production of polylactic acid. Optically pure lactide includes L-lactide and D-lactide. At present, the synthesis of commercially available L-/D-lactide is performed by the metal salt catalyst (e.g. stannous octoate, stannous chloride, etc., referring to U.S. Pat. No. 5,053,522). This method has the advantages: the metallic tin catalysts easily contaminate the products, not easy to regenerate, and possibly cause pollution to the environment, etc.